This invention relates to a filter circuIt arrangement comprising a frequency discriminator for producing an output signal representative of the deviation (if any) of the frequency of an input signal thereto from the resonant frequency of a resonant circuit included in said discriminator, said resonant frequency being a function of the value of an electrical parameter of an element of the resonant circuit and being adjustable by applying an adjusting signal to an adjusting signal input of said resonant circuit to thereby adjust the value of said electrical parameter, a coupling extending from the output of the discriminator to the adjusting signal input for applying a signal to said adjusting signal input dependent on said deviation to adjust said resonant frequency towards that of the input signal, a filter circuit the position(s) of the pole(s) and/or zero(s) of the frequency response characteristic of which is/are a function of the value of an electrical parameter of an element of said filter circuit and is/are adjustable by applying an adjusting signal to an adjusting signal input of said filter circuit to thereby adjust the value of said electrical parameter of the element of said filter circuit, and a coupling from an output of the discriminator to the adjusting signal input of said filter circuit for applying an adjusting signal thereto to adjust said position(s) with the adjustment of the resonant frequency of the resonant circuit.
An arrangement of this general kind is known from e.g. GB-A-1421093, which corresponds to U.S. Pat. No. 3,947,856 (12/14/76) In this known filter arrangement the resonant circuit and the filter circuit are each in the form of a gyrator circuit having a variable capacitor connected across each port. The adjusting signals control the values of the variable capacitors (which are in the form of so-called varicaps) and hence the resonant frequencies (which are functions of said values). If several such arrangements are each constructed in integrated circuit form the adjustment of the resonant frequencies can compensate for the inevitable production spreads which occur in these frequencies from circuit to circuit. However it has been found that satisfactory results are difficult to obtain with this known arrangement if the resonant frequencies are in other than a simple integer ratio to each other. If the resonant frequencies are in a simple integer ratio of, for example, 2:1 no problem arises as, for example, the values of the capacitors in one resonant circuit can simply be made double that in the other, e.g. by connecting pairs of identically controlled identical capacitors in parallel in one circuit. (Making varicaps on an integrated circuit of equal value is a relatively easy thing to do whereas constructing pairs of single capacitors to have a specific capacitance ratio other than 1:1 is extremely difficult). However, if the resonant frequencies are in a more complicated ratio of, say, 1.7:1.0, each capacitor in one resonant circuit would have to be composed of seventeen parallel-connected capacitors whereas its counterpart in the other resonant circuit would have to be composed of ten parallel-connected capacitors, which would obviously occupy a large area of the surface of the IC chip. On the face of it an alternative way of achieving the required capacitance ratio would be to arrange that the adjusting signal applied to the varicaps of one resonant circuit be suitably attenuated relative to that applied to the varicaps of the other. However, this has been found to be unworkable in practice due to the non-linear relationship between the capacitance of a reverse-biassed p-n junction and the value of the reverse bias. It is an object of the present invention to mitigate this problem.